The invention relates to a method for producing soybean meal, which is essentially free of bitter taste. The soybean is probably the best known and most versatile oil crop. In recent years it has gained considerable nutritional importance as a source of vegetable fat and a source of protein. The content of oil and protein varies, depending on the origin of the beans. The oil content lies generally between 15 and 21% whereas the protein content is 35% and more. The soybean is also the chief raw material for the production of vegetable phosphatides, e.g., lecithin, which are used in foods and in other technological areas.
For the processing, the beans are first precleaned where everything which is larger than the beans is removed. Subsequently, everything which is smaller than the soybeans is removed, as well as iron particles which are removed by magnetic cleaning devices. If the water content is more than 10%, drying to a water content of 8 to 10% usually is carried out before the beans are crushed by way of grooved crushing rolls. The crushed soybeans are then rolled on mills into small flakes as thin as possible. Then the flakes are fed to the extractors where soybean oil is extracted from the flakes by means of extraction agents. The residual meal, loaded with the extracting solvent, is removed from the extractors, freed of the solvent, dried, ground and cooled.
For soybeans, the oil-bearing seed predominantly processed by extraction systems today, the separation of solvent from extraction residues is of decisive importance since the value of the meal depends on it. Distillation or evaporation of the solvent by heat is carried out in a step called toasting, in order to render ineffective certain harmful components such as enzymes and to convert the protein into an easily digestible state. The meal, which is still solvent-moist, enters the dome of a toaster and passes downwardly through one after another of the individual levels, with the depth of the layer remaining the same at each level. Steam is blown into the toaster, which brings about the vaporization of the solvent and moistens the soy material additionally with the water of condensation. The following levels are equipped with indirect heating means. Here the actual toasting begins, which is preferably carried out at temperatures from 102.degree. to 105.degree. C. while predetermined moisture contents and times of stay are maintained. The effectiveness of the toasting process is evaluated by measuring the urease activity. This processing method is generally used today and in principle it is used in association with all extraction systems in the same general manner.
In order to achieve extraction as complete as possible, a looser packing of the small soy flakes--thus less bulk weight--has always been desirable in known processes. Likewise, any unecessary thermal stress before the extraction is to be avoided in order to preclude denaturation of the soy protein and thermal oxidation damage of the soybean oil which is not yet extracted. The separated meal still contains bitter components whose presence has always prevented the use of soybean meal for human food up to now.
It is an object of the present invention to produce a soybean meal which is largely free from bitter components, has a high content of digestible protein and is optimally free from the raw soybean oil and phosphatides.
As set forth herein, this object is achieved by a method wherein the soybean pieces in the form of crushed and rolled flakes, before extraction takes place, are first moistened, preferably by direct application of steam under fast mixing, to a water content of 12 to 25 weight percent, are then subjected to a heat treatment at temperatures of 90.degree. to 120.degree. C. under a slow, compacting mechanical movement sufficient to achieve a minimum bulk weight of at least about 400 kg/m.sup.3 and are subsequently freed of the raw soybean oil by extraction with a non-polar extraction solvent. The residual solvent is then removed by a conventional drying or toasting step.
The soy pieces are preferably moistened to a water content of 14 to 20 weight percent, especially about 15-16% by weight, under a steam atmosphere. Both the moistening and heat treatment are appropriately carried out in a tower which has plates with steam coils at several levels. The upper entry portion may be provided with apparatus for the moistening. The slow mechanical movement of the material which occurs during the heat treatment can be continued during an initial drying after the heat-treatment and also during the extraction. The initial drying takes place to such an extent that the usual water content for extraction is reached, e.g. between 8 and 14 weight percent, preferably about 12% by wt.
Good results are obtained especially if, contrary to prevailing opinion, the soybeans are slowly mechanically moved during the heat treatment until a bulk weight of at least 400 kg/m.sup.3, preferably over 500 kg/m.sup.3, is reached and the soy material with this high bulk weight is then used in the extraction. For this purpose, stirring paddles can be installed in the mentioned tower which move the soy material over the plates at various levels or in tiers, where the soy material falls from each level through tubes or the like to the next lower level. In order to effectively increase the bulk weight, i.e. so as to compact the soybean flakes to a higher density during the heat treatment step, it will be understood that a slow, gentle and compacting movement of a paddle mixer, stirrer or similar mixing means is required, for example, using a rotational speed of less than one rotation per second, i.e. less than about 60 r.p.m. Preferred rotational periods are about 2-6 seconds for each revolution, i.e. speeds of about 10 to 30 r.p.m.
In a preferred form of the invention, hydrocarbons containing 5 to 12 carbon atoms in the molecule are used, either singly or in a mixture, as the extraction solvent. Pentane, hexane, cyclohexane, heptane, nonane, octane and light benzine having a boiling range of 65.degree. to 85.degree. C. are examples of technically pure solvents which are especially useful for the extraction.
It proved to be favorable if the moistening and the heat treatment are carried out with small soybean flakes, which had been rolled in the crushing mills, because a treatment time of 10-45 minutes then suffices for the heat treatment.
Moistening may be accomplished with steam which is blown directly on the soy material. The steam, on the one hand, heats the soy material to the temperatures of the heat treatment and, on the other hand, moistens the soy material with water of condensation in the desired manner. This initial moistening and preheating of the soybean material is desirably carried out very rapidly, e.g. at mixing speeds above about 200 r.p.m. and within about 10 seconds or less.
In the method of the invention it is desirable to separate the residual extraction solvent from the soybean meal in a known meal toasting step.
The raw oil produced from the extraction phase was obtained in a form in which soybean oil and lecithin can be separated especially well and quantitatively. The lecithin yield is about 1.2 weight percent, with reference to the soybean charge.
Compared to a soybean meal which has been produced according to previously known methods, the great advantage of the method of the present invention resides on the recovery of a soybean meal with a low content of bitter components, so that it may be used as food intended for human consumption.
The special flake structure and the attainable higher bulk weight provide clear advantages for the storing and transportation of the meal due to better flowability and lower space requirements. Another advantage of the method of the invention arises from the notably increased yield and quality of the extracted products.
Compared to previously well known methods, the present invention provides an increase in the yield of extracted oil of 0.6% by weight with reference to the soybeans charged. The quantity of recovered lecithin is relatively large, and amounts of up to 1.2% by weight, with reference to the soybeans charged, are achieved.